32 ANSWERS TO PRACTICAL QUESTIONS 



Because it is equally distributed within and without our 

 bodies. The pressure on a person of ordinary size is about 16 

 tons. 



1 6. Is a bottle empty when filled with air? 

 No ; because we must empty the air out before we can fill 

 the bottle with anything else. 



1 8. How does the variation in the pressiire of the air affect 

 those who ascend lofty mountains? 



The outward pressure is there partly removed, and the 

 inner pressure remaining the same, the blood is often forced 

 through the ears, nostrils, etc. When one descends into a 

 deep mine the conditions are reversed : the outer pressure be- 

 comes in excess of the inner; severe pain is felt in the ear- 

 drum, and ringing noises in the head become almost intoler- 

 able. These, however, disappear after a time, where the 

 equilibrium between the internal and external pressure is 

 restored. It is said that Humboldt ascended where the mer- 

 curial column fell to 14 inches, and descended in a diving-bell 

 where it rose to 45 inches thus making a variation of 3 1 inches, 

 or a difference of 31,000 Ibs. pressure on his body. 



If the atmosphere in a diving-bell were of the same density 

 as that at the surface of the earth, how deep in the water 

 would it be necessary to sink the bell in order to reduce the 

 volume of the air one-half, or, hi other words, for the bell to 

 half fill with water? Ans. 34 feet. 



How 'near would the bell be filled at a depth of 1,020 

 feet. Ans. 29 / 3 o. 



If the bell were then raised, would the water stay in till it 

 reached the siirface f 



The elasticity of the air would cause it to gradually expand 

 and drive out the water as it rose. 



ACOUSTICS. 



14. i. Why cannot the rear of a long column of soldiers 

 keep time fo the music ? 



